A. Field of the Invention
The invention relates generally to the field of imaging devices and, more specifically, to driver circuitry for digitally operated imaging devices.
B. Discussion of the Prior Art
Traditional techniques of introducing a printed image onto a recording material include letterpress printing, gravure printing and offset lithography. All of these printing methods require a plate, usually loaded onto a plate cylinder of a rotary press for efficiency, to transfer ink in the pattern of the image. In letterpress printing, the image pattern is represented on the plate in the form of raised areas that accept ink and transfer it onto the recording medium by impression. Gravure printing plates, in contrast, contain series of wells or indentations that accept ink for deposit onto the recording medium; excess ink must be removed from the surface of the plate by a doctor blade or similar device prior to contact between the plate and the recording medium.
In the case of offset lithography, the image is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry lithographic printing system, the plate is simply inked and the image transferred onto a recording material; usually, the plate first makes contact with a relatively soft intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other copying medium. In typical rotary press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic printing system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
The plates for an offset press are usually produced photographically. In a typical negative-working subtractive process, the original document is photographed to produce a photographic negative. This negative is placed on an aluminum plate having a water-receptive oxide surface coated with a photopolymer. Upon exposure to light or other radiation through the negative, the areas of the coating that received radiation (corresponding to the dark or printed areas of the original) cure to a durable oleophilic state. The plate is then subjected to a developing process that removes the uncured areas of the coating (i.e., those which did not receive radiation, corresponding to the non-image or background areas of the original), and these non-cured areas become oleophobic and/or hydrophilic.
A number of difficulties attend both the platemaking and ink-transfer stages of printing. The photographic process used to produce conventional plates is time-consuming and requires a facility and equipment adequate to support the necessary chemistry. To circumvent this process, practitioners have developed a number of electronic alternatives to plate imaging, some of which can be utilized on-press. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in patterns representative of the images to be printed. Such imaging devices include sources of electromagnetic-radiation pulses, produced by one or more laser or non-laser sources, that create chemical changes on plate blanks (thereby eliminating the need for a photographic negative); ink-jet equipment that directly deposits ink-repellent or ink-accepting spots on plate blanks; and spark-discharge equipment, in which an electrode in contact with or spaced close to a plate blank produces electrical sparks to physically alter the topology of the plate blank, thereby producing "dots" which collectively form a desired image.
Two types of spark-discharge imaging equipment are currently in use. Both are amenable to digital control. In the older version of the technology, often referred to as "electroerosion", a stylus electrode is in constant contact with the surface of a plate blank. Suitable plate blanks include an oleophilic, or ink-accepting, plastic substrate (e.g., Mylar plastic film), a thin coating of aluminum thereover, and an overcoating containing conductive particles (e.g., graphite) that act as a lubricant to reduce scratching of the aluminum by the stylus; see, e.g., U.S. Pat. No. 4,596,733.
The electroerosion stylus is caused to move across the surface and receives relatively low-voltage electrical pulses in accordance with incoming picture signals. The resultant current flow between the stylus and the plate is, by design, sufficient to erode away the thin aluminum layer and the overlying lubrication coating, thereby exposing the underlying oleophilic layer. The pattern of applied pulses corresponds to the printed portions of the document. After the image is applied to the plate blank in this manner, the remaining coating is washed away, leaving the imaged plate with a pattern of hydrophilic (non-image) metal areas and oleophilic (image) areas.
This method of making lithographic plates is disadvantaged in that the electroerosion process works only on plates whose conductive surface coatings are very thin, resulting in somewhat fragile plates capable of a relatively small number of production runs. Furthermore, contact between the stylus and the surface of the plate frequently produces scratches that degrade the final image, because the scratches produce inadvertent or unwanted image areas on the plate which, in turn, print unwanted marks on the final copies. Again because of the contact between stylus and plate, the electroerosion process tends to be rather slow.
The second type of spark-discharge imaging equipment relies on electric arcs or plasma-jet discharges to ablate one or more surface layers of a lithographic plate blank to produce a similar pattern of oleophilic and oleophobic (or hydrophilic) layers. As in electroerosion systems, the discharge devices scan the plate blanks in accordance with digital picture signals to produce the necessary plate topologies. Suitable apparatus for performing this type of imaging are described in U.S. Pat. No. 4,911,075 (commonly owned with the present application and hereby incorporated by reference), U.S. Pat. No. 5,062,364 (commonly owned with the present application and hereby incorporated by reference), and a PCT application filed in the U.S. Patent and Trademark Office on Sep. 28, 1990 entitled "Plasma-Jet Imaging Apparatus and Method" and assigned Ser. No. 90/05546 (also commonly owned with the present application and hereby incorporated by reference). Hereafter, the term "spark discharge" will be used to refer both to production of electrical arcs and plasma-jet discharges.
Suitable plate constructions, designed for use with the latter type of imaging equipment, are described in U.S. Pat. No. 4,911,075 and U.S. Pat. Nos. 5,109,771 and 5,052,292. These plates contain, at a minimum, a conductive metal layer of greater thickness than that used in electroerosion, and a second layer underlying the metal layer, the metal and underlying layers having different affinities for ink and/or water. Because of the thickness of the metal layer, the spark discharges must be more powerful than those associated with electroerosion.
Use of a relatively thick metal imaging layer confers two key advantages. The first is high imaging accuracy. In a noncontact imaging system, reproduction accuracy depends on the ability to prevent the discharge from wandering as it travels from its source to the surface of the plate. This ordinarily requires a high field gradient between the discharge source and the point on the plate that is to be imaged. The strongest part of the field on the plate, to which the discharge is most strongly attracted, occurs at the point precisely opposite the discharge source. However, the strength of the field at this point must be sufficiently greater than the strength at any other point to overcome the inherently random nature of the discharge. The stronger the gradient, the faster the field strength will diminish as the path from source to plate deviates from the normal. Accordingly, high output voltage creates a strong gradient, which in turn favors straight-line discharge travel by emphasizing the recession of the plate field strength in all directions away from the normal.
Second, high-energy discharges permit use of refractory materials in the plates. By employing strong surface and substrate layers, it is possible to produce lithographic plates that offer longer performance lifetimes than those of the prior art.
To exploit the capabilities of non-contact spark-discharge equipment, it is necessary to provide support circuitry capable of satisfying a number of demanding and potentially conflicting criteria. Because one advantage of non-contact systems is the possibility of high imaging speed facilitated by lack of contact between the imaging devices and the plate surface, the driver circuitry should switch rapidly in order to take advantage of this feature. In addition, because of the high voltage levels necessary to cause metal ablation and straight-line spark travel, the delivered pulses must have an extremely fast rise time. They should also decay very rapidly after they are applied to prevent unintended additional discharges. Finally, because the size of an image spot depends primarily on the applied discharge energy, the pulse width (and, preferably, the amplitude) should be controllable and relatively precise.